1. Field of the Invention
The present invention relates to a transistor provided with an amplifying function and a switching function, and to an integrated circuit using the transistor.
2. Related Background Art
Transistors are roughly classified into two types: a bipolar transistor that operates with actions of carriers of both an electron and a hole; and a unipolar transistor that operates with an action of a carrier of either an electron or a hole.
For example, as to a field effect transistor, a voltage is applied to a semiconductor through a gate electrode through a gate insulating film to induce charge in an interface between the gate insulating film and the semiconductor, thereby forming an inversion layer (channel) on a surface of the semiconductor, and thus, electrical conduction is established between a source and a drain. That is, the resistance of the semiconductor is changed due to a gate voltage to thereby change a current that flows between the source and the drain.
As described above, the field effect transistor is made to operate by making the Fermi level of the semiconductor fluctuate due to the gate voltage. Thus, when the voltage applied to the gate electrode fluctuates, the current that flows through the transistor inevitably fluctuates under the operation principle.
Further, when the field effect transistor is in a conductive state (on state), an electric field perpendicular to a channel length direction (moving direction of carriers) is formed in the channel due to the gate voltage. However, the electric field in such a perpendicular direction is one of serious causes of hot carrier injection to the gate insulating film.
When injected into the gate insulating film, hot carriers are trapped by the gate insulating film to form a trapping level, or disconnect bonding of the interface between the gate insulating film and the semiconductor layer to form an interface level, which causes fluctuation in a threshold voltage of the transistor. When the threshold voltage fluctuates, for example, a timing of switching of the transistor varies, or a drain current fluctuates, which becomes a cause of malfunction of a circuit.
The present invention has been made in view of the above, and therefore has an object to provide a transistor in which a current that flows between a source and a drain can be kept constant even though a gate voltage is changed and which is based on the operation principle different from that in a conventional transistor.
Further, the present invention has another object to eliminate deterioration due to hot carrier injection.
A transistor according to the present invention includes: a semiconductor layer provided with a source region, a drain region, and a channel region that serves as a path of a current (carriers) between the source region and the drain region; an insulating film functioning as a gate insulating film that contacts with the semiconductor layer; and a gate electrode that overlaps with the semiconductor layer through the insulating film, and is characterized in that: another semiconductor region that contacts with the channel region is provided in the semiconductor layer; and the gate electrode is provided so as not to overlap with the channel region but to overlap with the semiconductor layer and so as to overlap with the semiconductor region that contacts with the channel region.
That is, in the transistor according to the present invention, the gate electrode is provided so as not to apply a gate voltage to the channel region provided in the semiconductor layer, and the semiconductor region (inversion layer formation region) for forming an inversion layer due to an electric field of the gate electrode is provided in the semiconductor layer so as to contact with the channel formation region.
Further, in the transistor according to the present invention, a semiconductor that constitutes the semiconductor layer of the transistor is formed of a semiconductor of a single element such as Si or Ge, a compound semiconductor made of GaAs, InP, SiC, ZnSe, or GaN, or a semiconductor formed of mixed crystal such as SiGe or AlxGaAs1xe2x88x92x. Further, the crystalline structure of the semiconductor may be any of a monocrystalline structure, polycrystalline structure, microcrystalline structure, and amorphous structure.
For example, a silicon wafer, an amorphous silicon film deposited by a CVD method, a sputtering method, or the like, or a polycrystalline silicon film obtained by crystallizing such an amorphous silicon film can be used as the semiconductor layer.
Further, the regions such as the channel region and the source region which are formed in the semiconductor layer, are each imparted with an appropriate conductivity type in accordance with the conductivity type of the transistor (n-channel type or p-channel type) although this is described later.
In the case where the semiconductor that constitutes the semiconductor layer is formed of silicon or germanium, as a dopant added into the semiconductor layer for imparting conductivity, an impurity that functions as an acceptor, such as B (boron), Sn, or Al, is added in the case of forming a p-type semiconductor region while an impurity that functions as a donor, such as P (phosphorous), As, or Sb is added in the case of forming an n-type semiconductor region.
The transistor according to the present invention which has the above-described structure is the same as a field effect transistor in the point that: a voltage is applied to the semiconductor via the gate electrode through the gate insulating film to induce carriers (electrons or holes) on the semiconductor surface due to electrostatic induction; and the transistor is made to operate with the action of carriers that are either electrons or holes.
However, the completely different point of the transistor according to the present invention from the conventional field effect transistor is that the gate voltage is applied not to the channel region but to the semiconductor region that contacts with the channel region through the gate insulating film to induce carriers, thereby forming the inversion layer.
In order to set the transistor according to the present invention in an on state (conductive state), a voltage equal to or more than a threshold voltage is applied to the semiconductor region via the gate electrode to thereby form the inversion layer.
Charge induced by the inversion layer moves to the channel region. As a result, the Fermi level of the channel region moves so that a potential barrier between the source region and the channel region becomes low. Then, the charge can climb over the barrier and move from the source region to the drain region, and thus, a drain current flows.
As described above, the transistor according to the present invention can operate in the same manner as the conventional transistor although this is described below in detail. Further, the transistor according to the present invention can be applied to various integrated circuits in which a conventional MOS transistor or thin film transistor is used. For example, the transistor according to the present invention can be applied to various integrated circuits such as memories like an SRAM and a DRAM, a processing circuit, and an image sensor using a CMOS transistor.
In addition, the transistor according to the present invention can now be applied to an active matrix display using liquid crystal or organic electroluminescence in which a TFT is used.
Further, as described above, the transistor according to the present invention does not have a characteristic that the charge is induced on the semiconductor surface due to the electric field to form the inversion layer (channel), thereby lowering the barrier between the source region and the channel region, but has a characteristic that carriers are injected into the channel region from the outside to change the Fermi level of the channel region, thereby lowering the barrier between the source region and the channel region.
Accordingly, in the present invention, the barrier between the source region and the channel region does not influence fluctuation of the gate voltage, and thus, the drain current does not fluctuate even though the gate voltage fluctuate, and is kept constant.